I think a lot about whether to federate or not depends on your organization. In our agency, with no enterprise license and more users than named-users, I do not plan to federate. From my understanding from recent training and tech sessions, if you federate, Portal takes over much of the security aspects over ArcGIS Server, and if you publish something in Server, it is automatically available on Portal and assigned to the named-user. This is way over simplifying things, but for us (once we upgrade to 10.5), we will probably have Server and Portal on the same machine, underated, and using Server's folder security. We use Active Directory for our user store and will try Integrated Windows Authorization (IWA) if it doesn't mess up with any of our other security and access.
With that said, and what Robert talked about as far as release timing, I have done initial tests on creating a WAB appid with Portal 10.5 and the latest WAB and it started up with no issues. I haven't gone beyond that with testing Portal.
